The present invention relates to optical devices. In particular, the present invention relates to optical devices that include optical interaction regions, such as optical filters and optical modulators.
Acousto-optic tunable filters (AOTFs) are electrically-tunable optical filters. Wavelength tuning is accomplished by varying the surface acoustic wave frequency applied to the AOTFs. AOTFs are useful for optical filtering and add-drop multiplexing in wavelength division multiplexing (WDM) optical transport systems. WDM is an optical transport technology that propagates many wavelengths in the same optical fiber, thus effectively increasing the aggregate bandwidth per fiber to the sum of the bit rates of each wavelength. Dense Wavelength Division Multiplexing (DWDM) is a technology that implements WDM technology with a large number of wavelengths. DWDM is typically used to describe WDM technology that propagates more than 40 wavelengths in a single optical fiber.
As the number of wavelengths increases, the channel width and channel spacing decreases. To achieve the required channel width and channel spacing in DWDM communication systems, high quality, high performance optical filters are required. In order to function properly, these optical filters generally must exhibit low loss and narrow band transmission characteristics over the wavelength spectrum of 1.3 xcexcm to 1.55 xcexcm. These filters generally must also have good mechanical properties and must be stable in typical operating environments.
AOTFs are particularly advantageous for use in WDM optical transport systems because they can achieve narrow passbands and broad tuning ranges. In fact, an AOTF can have a tuning range that is substantially the entire wavelength range of an optical fiber communication system, which can typically be approximately from 1.3 xcexcm to 1.6 xcexcm. Also, AOTFs have the unique capability of simultaneous multi-channel filtering. By simultaneous multi-channel filtering we mean that an AOTF can select several wavelength channels simultaneously by applying multiple acoustic wave signals. In addition, AOTFs can be configured as add-drop multiplexers. Add-drop multiplexers are used in WDM optical transport systems for adding and dropping one or more channels while preserving the integrity of the other channels.
AOTFs include a narrowband polarization converter that is positioned between an input and an output polarizing element. The polarization converter changes one polarization mode to an orthogonal polarization mode. Light having a wavelength range within the passband of the filter propagates through the input polarizing element and then is converted to an orthogonal state of polarization. The converted light then propagates through the output polarization element.
The degree of polarization transformation depends on the magnitude of the polarization conversion, which is a function of the applied acoustic power density. However, the polarization converter is inoperative outside of the passband of the filter. Light having a wavelength range outside of the passband does not get converted by the polarization converter and, therefore, is blocked from propagating through the AOTF.
Known AOTFs have several practical limitations that have prevented them from being used in commercial WDM optical transport systems. For example, known AOTFs have relatively wide channel bandwidth and have relatively poor out-of-band signal suppression. Also, known AOTFs that are configured as multi-wavelength add/drop multiplexers experience coherent beating between multiple drive frequencies when performing multi-wavelength add/drops. This can lead to undesirable wavelengths being included when performing multi-wavelength add/drops.
The present invention relates to optical devices that include multi-segment optical interaction regions. In one embodiment, the present invention relates to AOTF devices having long interaction lengths. An AOTF according to the present invention divides the optical interaction region into a plurality of segments, such that the total combined length of the plurality of segments is the desired interaction length. In one embodiment, the plurality of segments comprises segments that are positioned adjacent to each other in numerous folded configurations.
AOTFs according to the present invention have a relatively low aspect ratio. By aspect ratio we mean the ratio of the physical length of the device to its physical width. Devices having low aspect ratios generally are more physically robust and axe generally easier to package. In addition, an AOTF according to the present invention can use smaller heaters or thermoelectric coolers and less complex temperature controllers compared with devices having higher aspect ratios. In one embodiment, AOTFs according to the present invention have a relatively high yield because the materials they are fabricated from are generally more uniform and generally the fabrication tolerances ate less demanding for devices with low aspect ratios. In one embodiment, devices with low aspect ratios are less expensive to manufacture because more or the devices can occupy a given sized substrate compared with devices having standard or higher aspect ratios
Accordingly, in one aspect, the present invention is embodied in an acousto-optic tunable filter that includes a polarization beamsplitter for receiving an optical signal at a first optical input. The polarization beamsplitter generates a first and a second polarized optical signal at a first and a second optical output, respectively. In one embodiment, the polarization beamsplitter is formed in a substrate. In another embodiment, the polarization beamsplitter is a discrete planar device. In other embodiments, the polarization beamsplitter is a prism or other known polarization splitter device. In one embodiment, the first polarized optical signal is orthogonally polarized relative to the second polarized optical signal.
The acousto-optic tunable filter also includes a first optical interaction region having a first and a second optical waveguide optically coupled to the first and the second output of the polarization beamsplitter, respectively. The first optical interaction region also includes a first acoustic wave generator for generating acoustic waves in the first and the second optical waveguides. In one embodiment, the first optical interaction region is formed in a substrate. In another embodiment, the first optical interaction region is a discrete planar device. In another embodiment, the first optical interaction region includes a first and a second segment that are physically separate.
The acousto-optic tunable filter further includes a second optical interaction region having a third and a fourth optical waveguide optically coupled to the first and the second optical waveguide of the first optical interaction region, respectively. The second optical interaction region also includes a second acoustic wave generator for generating acoustic waves in the third and the fourth optical waveguides. The second optical interaction region is non-collinear relative to the first optical interaction region, thereby reducing the aspect ratio of the acousto-optic tunable filter.
In one embodiment, the second optical interaction region is formed in a substrate. In another embodiment, the second optical interaction region is a discrete planar device. In another embodiment, the second optical interaction region includes a first and a second segment that are physically separate. In one embodiment, the first optical interaction region and the second optical interaction region are discrete planar devices that are formed in a first and second physically separate substrate, respectively. In another embodiment, the first optical interaction region and the second optical interaction region are positioned adjacent to each other in a folded configuration. In yet another embodiment, the first optical interaction region is positioned in a non-parallel configuration relative to the second optical interaction region.
In one embodiment, the third and the fourth optical waveguides are optically coupled to the first and the second optical waveguides, respectively, with a first and a second optical fiber. The first and the second optical fibers can be positioned in a V-groove block.
In another embodiment, the third and the fourth optical waveguides are optically coupled to the first and the second optical waveguides with a fifth and a sixth optical waveguide. In one embodiment, the fifth and the sixth optical waveguides are curved. In another embodiment, the fifth and the sixth optical waveguides do not cross.
In yet another embodiment, the fifth and the sixth optical waveguides are ridge waveguides. In still another embodiment, the fifth and the sixth optical waveguides are channel waveguides. In one embodiment, the fifth and the sixth optical waveguides are disposed on discrete substrates.
The acousto-optic tunable filter also includes a polarization beam combiner having a first and a second input optically coupled to the third and the fourth optical waveguide of the second optical interaction region, respectively. The polarization beam combiner generates a substantially mode-converted optical signal at a first optical output in response to the acoustic waves generated by at least one of the first and the second acoustic wave generators. In one embodiment, the polarization beam combiner generates a non-mode-converted optical signal at a second optical output.
In one embodiment, the substantially mode-converted optical signal is phase-matched to the acoustic waves generated by at least one of the first and the second acoustic wave generators. In another embodiment, the substantially mode-converted optical signal comprises a wavelength that is inversely proportional to the frequency of the acoustic waves generated by at least one of the first and the second acoustic wave generators. The phase of the acoustic waves generated by the second acoustic wave generator is adjusted such that polarization mode conversion in the second optical interaction region is substantially coherent with polarization mode conversion in the first optical interaction region.
In one embodiment, the acousto-optic tunable filter also includes a third optical interaction region having a fifth and a sixth optical waveguide optically coupled to the third and the fourth optical waveguide, respectively. The third optical interaction region includes a third acoustic wave generator for generating acoustic waves in the fifth and the sixth optical waveguides. In another embodiment, the third optical interaction region is non-collinear relative to at least one of the first and the second optical interaction regions, thereby reducing the aspect ratio of the acousto-optic tunable filter.
In yet another embodiment, a phase of the acoustic waves generated by the third acoustic wave generator is adjusted such that polarization mode conversion in the third optical interaction region is substantially coherent with polarization mode conversion in at least one of the first optical interaction region and the second optical interaction region.
In one embodiment, the acousto-optic tunable filter optically processes a WDM optical signal in a WDM optical communication system. In another embodiment, the polarization beamsplitter, first optical interaction region, second optical interaction region, and polarization beam combiner are integrated on a single substrate. In yet another embodiment, at least one of the polarization beamsplitter, first optical interaction region, second optical interaction region, and polarization beam combiner comprises a discrete planar device. The discrete planar device can be formed on a physically separate substrate. The polarization beamsplitter can be a prism or other known polarization splitter device.
In another aspect, the present invention is embodied in a multi-segment acousto-optic interaction region. The multi-segment acousto-optic interaction region includes a first optical interaction region having a first optical waveguide with a first optical input and a first optical output. The multi-segment acousto-optic interaction region also includes a first acoustic wave generator for generating acoustic waves in the first optical waveguide.
The multi-segment acousto-optic interaction region includes a second optical interaction region that is non-collinear relative to the first optical interaction region. The second optical interaction region includes a second optical waveguide having a second optical input that is optically coupled to the first optical output of the first optical interaction region. The second optical interaction region also includes a second acoustic wave generator for generating acoustic waves in the second optical waveguide. The second optical interaction region generates a substantially mode-converted optical signal in response to the acoustic waves generated by at least one of the first and the second acoustic wave generators.
In another embodiment, the multi-segment acousto-optic interaction region includes a third optical interaction region. The third optical interaction region includes a third optical waveguide having a third optical input that is optically coupled to the second optical output of the second optical interaction region. The third optical interaction region also includes a third acoustic wave generator for generating acoustic waves in the third optical waveguide. In one embodiment, the third optical interaction region generates a substantially mode-converted optical signal in response to acoustic waves generated by at least one of the first, second, and third acoustic wave generators. In another embodiment, the third optical interaction region is non-collinear relative to at least one of the first and the second optical interaction regions.
In one embodiment, the second optical input of the second optical waveguide is optically coupled to the first optical output of the first optical interaction region by an optical fiber. In another embodiment, the second optical input is optically coupled to the first optical output by an optical waveguide. The optical waveguide can be a ridge optical waveguide. The optical waveguide can be a channel optical waveguide.
In another aspect, the present invention is embodied in a method of mode-converting an optical signal. The method includes generating acoustic waves in a first optical interaction region. The method also includes propagating the optical signal through the first optical interaction region. The method further includes generating acoustic waves in a second optical interaction region that is non-collinear relative to the first optical interaction region. The method further includes propagating the optical signal through the second optical interaction region, thereby substantially mode-converting the optical signal in response to the acoustic waves generated in the first and the second optical interaction regions.
In one embodiment, the first and the second optical interaction regions are positioned adjacent to each other in a folded configuration. In another embodiment, the first and the second optical interaction regions are positioned adjacent to each other in a non-parallel configuration. In another embodiment, the method further includes reflecting the optical signal propagating through the first optical interaction region into the second optical interaction region. In another embodiment, the method further includes propagating the optical beam from the first optical interaction region into the second optical interaction region with an optical fiber.